The present invention relates to control of emissions from Ocean Going Vessels (OGVs) at berth or anchored in port and in particular to the capture of exhaust gases from the OGVs to allow subsequent processing.
OGVs at berth or anchored in port are a significant source of air pollution from the exhaust gases of their auxiliary power sources. The OGVs have auxiliary diesel engines and auxiliary boilers which normally remain in operation while the OGV is at berth or anchored. The auxiliary engines drive generators which provide power for ballast and other pumps, onboard motors, shipboard lighting and air conditioning, communications equipment, and other housekeeping functions. The auxiliary boilers are used to keep the bunker fuel used to feed the main engines warm. If allowed to cool, this fuel becomes so viscous it is difficult or impossible to pump. Heat from the boilers is also used for other shipboard functions.
The boilers and auxiliary engines are a source of nitrogen dioxide (NOx), sulfur dioxide (SOx), particulate matter (PM), and volatile organic compounds (VOCs), all of which are air pollutants. Left untreated, the OGVs are one of the most significant sources of air pollution in the vicinity of ports throughout the world.
The OGV discharge the exhaust from the auxiliary engines and auxiliary boilers, as well as other less significant pollutant sources such as the galley and incinerators, through exhaust pipes clustered within an exhaust stack which runs from the engine room up through the OGV's decks and continues some distance above the top deck. U.S. Pat. No. 7,258,710 for “Maritime Emissions Control System,” assigned to the assignee of the present invention, describes a maritime emissions control system which may be transported by barge or vessel to an OGV near or within a harbor. The maritime emissions control system uses a bonnet (or umbrella-like device) which is lifted up above the top of the OGV exhaust stack and then lowered over the entire exhaust stack of the OGV and sealing around the exhaust stack perimeter, thereby capturing exhaust otherwise released to the environment. The '710 patent further discloses processing the exhaust flow from the OGV to reduce emissions. The '710 patent is herein incorporated in its entirety by reference.
The top of the stack is usually the highest part (other than antennas) of the OGV. The stack of an OGV is typically 15 to 30 feet on a side and often rectangular, but it can be round or oval or some other shape. The stack side walls are usually vertical or slightly tapered, but sometimes have sharp lips protruding from the side, railings near the top, or other features which depart from smooth sides, and the stacks frequently include air intakes for the engine room and other interior spaces at some point below the stack rim. The individual exhaust pipes exit through a recessed floor located a few feet below the top rim of the stack, and generally extend upward a small distance above the stack.
Unfortunately, there are some difficulties in positioning a bonnet over the OGV stack. The OGV's antennas are often close to the stack, wires are often attached to the rim of stack, the stack may be built into OGV superstructure, appendages reach from the rim of stack, and/or extreme stack dimensions.
Another issue is the difficulty in sealing the bonnet to the stack, which can result in escape of exhaust gas or ingestion of air through a gap between the vessel stack and the inner periphery of the bonnet in which case the bonnet will capture only a portion of the exhaust gas and simultaneously ingest significant volumes of atmospheric air reducing the efficiency of the processing unit. Tension devices intended to effect a seal between the bonnet and stack are difficult at best to implement on a rectangular surface with large flat sides, and bladder type devices are cumbersome and unreliable and require external apparatus such as an air blower or pressure vessel to inflate them. Even if an acceptable seal is achieved with a bonnet, wind forces and wind gusts “pump” the bonnet like a bellows once it is attached, and this action may force exhaust gas out around the perimeter while simultaneously creating over-pressure followed by under-pressure within the bonnet and consequently on the boiler exhaust, which is detrimental to boiler performance and can even cause flame extinguishment in extreme cases.
Another issue is the ingestion of non-exhaust gas flowing from vent pipes in the stack, gaps around each of the exhaust pipes where they exit the stack deck, gaps around the access door to the stack deck, and steam vents. Another issue is the bonnet's size and weight make using it on a barge potentially difficult.
Another issue is the “breathing” or volumetric changes to the inside volume of the bonnet caused by wind eddies and gusts, which often have the effect of forcing exhaust gas out the imperfect seal around the interface between the bonnet and the stack.
Another issue is the difficulty in attaching the bonnet to the stack during high winds because of the large bonnet surface area exposed to the wind. The surface of a bonnet sufficiently large to encompass the typical OGV exhaust stack is 20 to 30 feet on a side, usually square, rectangular, hexagonal, octagonal, or oval. Such a large surface, hanging from a crane or other placement device many tens of feet and often over 100 feet in the air, is frequently subjected to wind forces which cause the bonnet to sway with relatively large excursions, creating the danger of hitting and damaging antennas or other nearby OGV structures. This movement also makes alignment and attachment to the stack difficult if not impossible in windy conditions.
An additional drawback to the bonnet is its weight. Due to the size required to encompass the OGV stack, the bonnet and its actuating and support structure is heavy, requiring a substantial lifting capability for the crane or other placement device.
Yet another drawback is inherent in the large volume enclosed by the bonnet. The interior of the bonnet is the size of a room, so control of the vacuum within this bonnet which is necessary to ensure that gas is drawn out of the pipes rather than impeded from exiting each exhaust pipe, and of ensuring that over pressure will not force exhaust gas back down to the engine room through openings in the stack itself, is very difficult, exacerbated by variable exhaust flows and gusting wind.
Another issue is that known bonnet designs are fairly complex and may prove unreliable. Known bonnet designs include numerous motors, cables, and control devices to furl and unfurl or extend and retract and secure the bonnet to the stack, all of which add complexity, weight, and failure modes, and require that electrical, pneumatic, or hydraulic power be routed to the bonnet. Additionally, a large crane or similar lifting and placement device is required for the bonnet due to its weight and potential wind forces, which adds cost and structure needed to support the crane.